1. Field of the Invention
The present invention relates to a heterostructure manufacturing method and device and, more particularly, to the nanofabrication of heterostructure devices.
2. Description of the Prior Art
Rapidly developing nanofabrication technologies such as electron-beam (e-beam) and atomic-force-microscope (AFM) lithography and a variety of growth/synthesis techniques have enabled a number of material systems to exhibit mesoscopic phenomena. Some of these systems include metallic wires/rings, carbon nanotubes, and GaAs/AlGaAs heterostructures. Among these, the GaAs/AlGaAs system is the most intensively studied due to the long mean free path (le) of two-dimensional (2D) electrons in heterojunctions. As a result, it is possible with current technology to fabricate ballistic one-dimensional (1D) wires, where W<L<le and W and L are the lateral confinement dimension of the device, i.e., width and length, respectively. The lateral confinement can be accomplished by a number of methods, e.g., mesa-etch or split-gate approaches. However, due to mid-gap pinning of the surface Fermi level (Efs) in the GaAs/AlGaAs system, electrons are completely depleted in heterojunctions whose metallurgical width (Wm) is narrower than 0.5 μm or so. Recently, a number of variations on nanofabrication, e.g., AFM anodization and front-gate induction, have been attempted to further reduce device dimensions.
The 2D electrons in InAs quantum wells (QW's) are also known to have a long mean free path. In addition, InAs has a number of properties that are advantageous for nanofabrication and for studying low-dimensional physics. First, the surface Fermi level pinning position in InAs, Efs (InAs), is above the conduction band. This property makes it possible to fabricate isolated conducting wires with widths in nanometers, which suggests the possibility of fabricating a complex circuit with compact dimensions. Second, a small electron effective mass (0.023 m0) results in a large quantization energy, which is favorable for the observation of low-dimensional phenomena at higher temperatures. Finally, the relatively large Lande g-factor (g0=−15) is essential for studying spin related physics, e.g., Berry's phase resulting from the Rashba effect.
Although nanotechnology is a rapidly evolving field, current InAs nanofabrication is limited. For example, nanotechnology fabrication has been used to form InAs conducting wires and rings as well as double-barrier-tunneling dots with a minimum dimension of 30 nm using e-beam lithography and reactive-ion etching (RIE). However, these InAs devices have been limited in applicability for particular applications since the electron beam free path decreases as the wire width is reduced due to damage on the side wall and the top surface during dry etching and O2 ashing. Further, using prior nanofabrication techniques, ultra smallness is obtained at the expense of material quality, which is a drawback for practical applications.
Previous attempts of overcoming the aforementioned limitations of these manufacturing methods and resulting InAs devices, namely limitations due to side wall and top surface damage, includes the use of chemical etching which typically results in minimal damage. Using conventional chemical etching techniques and etchant solutions, a 20 nm depth wet etch can provide isolation of InAs wires. However, the marginal control of side wall roughness characteristic of this method, limits application of the method for deep submicron device fabrication. Generally speaking, the deeper the etched pattern, the rougher the side wall will be. As a result, chemical etching is rarely applied to nanofabrication. Moreover, lateral isolation by the physical isolation of an InAs quantum well may not be the ideal choice since the conducting electrons will be in close proximity to the surface states and may suffer from excess scattering.